Industrial scale chemical processes for preparation of isocyanates and the precursors required for preparation thereof are available. The processes can be operated batchwise, semicontinuously, continuously or in a combination of one of these three variants. The processes are endothermic or exothermic and can be conducted isothermally or adiabatically. According to the isocyanate to be prepared, the process can be conducted in the gas phase or in the liquid phase, with or without solvent, or in the melt. The workup and purification of the isocyanate thus obtained can be effected by one of the standard methods in the art, for example crystallization, washing, distillation, or in a combination of these workup methods.
The quality of a process for preparing an isocyanate is firstly defined by the content of unwanted by-products in the product of the process. Secondly, the quality of a process is defined in that the whole operation of startup and production in the target state until the shutdown of the process can be executed without technical production outage and without problems that would necessitate intervention in the operation, and that there are no losses of feedstocks, intermediates or end product.
Ideally, therefore, the industrial scale plants for performance of such preparation processes are designed such that the processes run in a robust manner in the event of appropriate quality of the auxiliaries and feedstocks used and correct choice of process parameters such as pressure, temperature, ratios of amount and concentrations of the auxiliaries and feedstocks, etc. This means that, in such continuously operated large-scale plants, there will ideally be no problems such as the formation of precipitates which can settle out in plant equipment or block pipelines.
In order to further enhance economic viability, the efficiency of the energy consumption of such industrial scale plants was enhanced even at an early stage. Energy consumption can be greatly reduced by sensible heat integration between the individual plant components of a production plant. The aim is to industrially implement the sensible utilization of excess energy, for example from an adiabatic process, as far as possible.
For example, WO 2004/056 761 A1 is concerned with heat integration in a process for preparing isocyanates by reacting amines with phosgene in the presence of inert organic solvents in a reactor and subsequent workup of the reaction mixture leaving the reactor. Heat integration is implemented in such a way that the removal of solvent is effected in a two- or multistage, preferably two-stage, distillative method and the solvent is removed in a first apparatus, preferably a distillation column, at 0.1 to 15 bar, preferably 0.5 to 3 bar, and in a second apparatus or further apparatuses, preferably likewise distillation columns, at 1 to 900 mbar, preferably 50 to 500 mbar, where the heat of condensation of the solvent vapor, also referred to as vapors, from the first apparatus is used for partial or complete evaporation of solvent in the second apparatus.
In the first column, solvent vapor is removed by distillation. The liquid component obtained as bottoms discharge from this column, upstream of or within the second apparatus, is expanded to the lower pressure level of the second apparatus and is fed to the second apparatus in which the rest of the solvent is removed. The transfer of energy in the vapors from the first apparatus to the liquid phase of the second apparatus can especially be effected using a heat exchanger, for example a crossflow apparatus, in which the condensing vapors bring about the evaporation of the liquid phase from the second apparatus of the second column. It is possible here to conduct the vapors and liquid phase in co- or countercurrent. The vapors may be condensed in the shell space or in the product space of the heat exchanger. The liquid in the second column can be withdrawn from the bottom, from a tray, from a liquid collector or from the feed. There is no description of any further use of energy remaining after the internal heat integration.
EP 1 854 783 A2 is also concerned with the preparation of isocyanates. Likewise described in this application is heat integration in the recovery of solvent. In a preferred embodiment of the process, the solvent-containing stream obtained in step d) and at least partly recycled in step e) is freed of residual amounts of phosgene in a special distillation stage. More preferably, in this distillative separation of the residual amounts of phosgene, the tangible heat in the solvent stream recovered is used wholly or partly as energy source for this separation step. This can be effected, for example, in that the feed into the distillation column heats the column bottoms via a heat exchanger. Since the solvent separated by distillation is normally obtained at a temperature of >100° C., but for generation of the solution of amine in the solvent for optimal phosgenation conditions should be at <50° C., the separation of the residual amounts of phosgene can thus be combined with simultaneous cooling of the solvent. There is no description of any further use of energy remaining after the internal heat integration.
European patent application EP 2 096 102 A1 is likewise concerned with the preparation of isocyanates. What is described is an integrated process for preparing di- and polyisocyanates of the diphenylmethane series (MDI; also referred to hereinafter as a mixture of methylene diphenylene diisocyanate —(OCN)C6H4—CH2—C6H4(NCO)— and polymethylene polyphenylene polyisocyanate —(OCN)C6H4—CH2—[C6H3(NCO)CH2—]XC6H4(NCO), where x is a natural number ≥1), in which the salt-containing wastewater obtained in the preparation of the precursor compounds of the di- and polyamines of the diphenylmethane series (MDA; also referred to hereinafter as a mixture of methylene diphenylene diamine and polymethylene polyphenylene polyamine) is electrolyzed to chlorine, and the chlorine thus obtained is converted to phosgene which is then used in the conversion of MDA to MDI. The invention thus relates to integration by mass transfer between the production plants for MDA and MDI. The patent application does not address heat integration.
International patent application WO 2008/148608 A1 describes the entire MDI process chain proceeding from the benzene starting material, which is converted to MDI by the steps of nitration, hydrogenation, condensation with formaldehyde and acid-catalyzed rearrangement and finally phosgenation. The patent application is based on the technical object of providing a process for preparing MDI, which enables use of benzene of a lower purity than is typically used in the prior art as starting material. One proposed way of achieving this object is to use benzene of a lower purity than is typically used in the prior art as starting material. This lower purity is characterized by a content of alkyl-substituted aromatics in the benzene of 500 to 5000 ppm. Heat integration between the individual process stages of the MDI process chain is not the subject of this application.
Industrial scale production plants for chemical products are nowadays constructed in a steel skeleton designed in order to counteract the lack of space that often prevails on existing chemical sites. It is advantageous to establish various stages in a process chain (for example in the preparation of various isocyanates, the stages of nitration, hydrogenation, optional further conversion of the hydrogenation product and phosgenation) at one site in an integrated production system in order to save logistics costs. The individual production plants in such an integrated production system are typically positioned in series. Building at close quarters also entails drawbacks that have to be minimized A disadvantage is the lack of space in the construction of individual production plants in an integrated production system; there can be problems with maintenance measures (for example setting up cranes). Safety aspects such as complying with the due safety margins between production plants have to be observed. The provision of auxiliaries and feedstocks is problematic in some cases owing to cramped roads and areas between the production plants. The necessarily greater distances from central tank farms (for auxiliaries, feedstocks, products) necessitate the use of longer pipelines with larger pumps.
Energy consumption for direct or indirect process heating and cooling by vapor, by comparison with other modes of energy consumption, especially of electrical energy, is a major cost pool in the operation of industrial scale production plants, and so measures for optimizing this energy consumption offer the greatest potential for savings. Heat integration based on a pinch analysis provides a remedy here. In this way, it is possible to ascertain, for example, the optimal configuration of the feed preheating/cooling for reduction of the energy demand of columns, of waste heat vapor production and of the use of heat pumps and vapor compression, or the generation of coolant in an absorption cooler. Residual heat available in a production plant can be used at other points in the same process, or for heating of nearby administration buildings. It is also possible to use residual heat no longer suitable for vapor generation to heat buildings. Relatively low feed temperatures are sufficient for heating of buildings, and so even residual heat no longer suitable for vapor generation is utilizable. It is also conceivable to use residual heat to generate coolant in an absorption cooler.
The current prior art processes for industrial scale preparation of isocyanates and precursors thereof generally succeed in preparing the desired products with high yield and in sensibly utilizing excess energy by heat integration within individual production plants. This is frequently accomplished in such a way that excess energy from an operation within a preparation process for a chemical product is used by means of a suitable apparatus (for example a heat exchanger) for generation of vapor, especially for generation of steam, and the vapor thus obtained is used in a heat-consuming operation in the same preparation process (in this regard see also the abovementioned examples from the patent literature). However, even after maximum exploitation of this “internal” heat integration (i.e. heat integration within the production plant for a chemical product), excess “residual energy” frequently still remains. Such residual energy can generally be used to generate only vapor at a comparatively low pressure and low temperature (also colloquially called “weak vapor”). Steam of this kind is then either merely utilized for heating of administration buildings or the residual energy is lost in the form of waste heat without any sensible utilization, or even incurs additional costs, for example through requirement of air coolers for “energy destruction”.
“True” heat integration with such residual energy has not been described to date in the prior art relating to preparation of isocyanates at the current state of knowledge, and has barely been described in general in the prior art:                International patent application WO 2013/083230 A1 discloses heat integration in a complex for preparation of vinyl chloride (VCM) from ethylene, hydrogen chloride and oxygen or from ethylene and chlorine via the isolated 1,2-dichloroethane (DCE) intermediate. VCM is of industrial significance as starting monomer for the preparation of polyvinyl chloride (PVC). In the process described, the heat obtained in the condensation of the top stream from a high boiler column for purification of the DCE intermediate is utilized to obtain low-pressure steam which then finds use, for example, for the operation of a stripping column for purification of the VCM or in various facilities in a downstream PVC plant. However, the individual constituents of a VCM complex or a VCM/PVC complex are not of comparable complexity to the individual production plants in an integrated isocyanate production system. The chemical processes are additionally entirely different than those in an integrated isocyanate production system. Owing to the variety of different chemical processes (examples include: chlorine production, carbon monoxide production, phosgene production, nitration including the provision of nitric acid required for the purpose, hydrogenation including the provision of the hydrogen required for the purpose, possibly condensation and rearrangement reactions, phosgenation), heat integration between individual production plants in an integrated isocyanate production system poses completely different challenges to the operators.        German patent application DE 10 2013 205 492 A1 discloses that “intrinsic vapor” at a temperature of typically 120° C. to 170° C. and an absolute pressure of typically 2 to 8 bar which is obtained in the preparation of vinyl acetate can be sent to other operations in an integrated works or can be used for heating of product pipelines or buildings. All these modes of utilization are described as disadvantageous; there is no disclosure of the kind of operations that could accept such vapor. It is an object of the application to make it possible to dispense with this mode of utilization of intrinsic vapor in favor of in-process utilization. A solution proposed is that of equipping particular columns in the process for preparation of vinyl acetate with random packings and hence enabling use of the intrinsic vapor for the energy supply of the columns thus equipped.        
Further improvements in energy integration in processes for preparing isocyanates, by means of which such residual energy can be sent to a sensible use in the preparation of chemical products, would therefore be desirable. Such improvements are desirable from sustainability and environmental aspects as well.